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Addition and Subtraction
Objectives
To add and subtract 20, 30, 40, 50 to or from 2-digit numbers using the 100 square grid.
Resources
Abacus Workbook 1 | Toy spider | 📄 Add or subtract 10s cards | 📄 Spider counters | 8 plastic cubes | 📄 100 square
Vocabulary
tens | ones | add | subtract
Today's Lesson
What to Get Ready
Cut out the 📄 Add or subtract 10s cards and the 📄 Spider counters.
Introduction
Before you start the lesson, watch this Recap Video with your student.
Access the 📄 100 square and place the toy spider on 43. Then move the spider down 2 squares.
Ask what Spider has done (counted on two 10s) and how much has been added (20). Record 43 + 20 = 63 on paper.
Emphasise that Spider doesn’t have to count in 1s!
Repeat, using Spider to show 26 + 30, 52 + 40 and 47 + 50.
Help your student to put up 1 finger for each 10 Spider adds, and then to count in 10s on their fingers.
For example, when adding 30 to 64, saying, ‘74, 84, 94’. Record the addition: 64 + 30 = 94.
Next place Spider on 43 and move 2 squares up the 1-100 grid.
Ask what Spider has done this time (counted back two 10s) and how much Spider subtracted (20). Record 43 – 20 = 23 on paper.
Repeat, this time showing 64 – 30, 84 – 30, 78 – 40.
Then, silently place Spider on 63 and then move down 3 squares. Ask your student to think and then to write the matching addition. They should have spotted that Spider added 30, so 63 + 30 = 93.
Repeat, this time Spider showing 85 – 40. Your student should then write the matching subtraction.
Main Activity
Spread out a set of the 📄 Add or subtract 10s cards, face down.
Play the following game with your student:
- Take a 📄 Spider counter each. There are lots of these in the resource so that this game can be played again and again if you wish.
- Colour your counter so it is a different colour to each other’s. Each player should then give their Spider 4 cubes. These are their Spider’s lives!
- Each of you should place your Spider on a number on the grid.
- Then one of you takes a card, for example ‘+30’.
- On the grid that player follows the card instruction using Spider. The other player should check it has been done correctly. Don’t replace the card on the table.
- BUT if Spider can’t do the calculation without falling off the grid, that player loses 1 of their Spider’s lives. They have to lose a cube!
- If Spider didn’t lose a life, record this calculation, for example: 42 + 30 = 72.
- The other player now takes a card, for example ‘-20’.
- If they can, they follow this instruction on the grid using their Spider, for example: 75 – 20 = 55. If they can’t follow the instruction without Spider falling off the grid, Spider loses a life.
- If they don’t lose a life, they should record the calculation.
- Continue taking it in turns to take a card until all the cards are gone or until one of the Spiders has lost all its lives.
Workbook
To help your student build upon their learning from today, please ask them to complete the activities on page 39 of Abacus Workbook 1.